1. Field of the Invention
The present invention relates to chemical process columns and, more particularly, to vapor diffuser assemblies for evenly distributing vapor flow relative to packing beds in such process columns.
2. History of the Prior Art
It is well known in the prior art to utilize various types of exchange columns in which a gas and a liquid come into contact with one another, preferably in a counter-current flow for purposes of mass or heat transfer, close fractionation and/or separation of feed stock constituents, and other unit operations. Efficient operation requires mass transfer, heat transfer, fluid vaporization and/or condensation, whereby one of the fluids can be cooled with a minimum pressure drop through a particular zone or zones of minimum dimensions defined in the area and volume thereof. These are pre-requisites for efficient operation and are necessary for close fractionation. For this reason, counter-current flow of vapor and liquid within such exchange columns, or process towers, have become established methods of such vapor liquid contact in the prior art. The actual vapor-liquid interface requires the utilization of a packing bed positioned within the column. Liquid is then distributed atop the packing bed in the most feasible manner while vapor is distributed beneath the packing bed in the lower region of the tower. In this manner, liquid trickling downwardly through the packing bed is exposed to, and in contact with, the vapor ascending therethrough for vapor-liquid contact and interaction.
It is well established that the configuration of the vapor flow in the lower region of the tower adjacent the vapor input nozzle is critical to uniform vapor-liquid contact. This is more so when structured packing beds are utilized as compared to trays. With trays, there is little concern about initial vapor distribution because pressure drop across a trayed column is high. For trayed tower with approximately 50 trays, a pressure drop on the order of 6 PSI (300 mmHg) is common in the prior art. This is, however, more than an order of magnitude greater than the kinetic energy generated by the incoming vapor. The velocity head of vapor entering the distillation column is often greater than 3 to 4 inches of water in refinery heavy oil fractionators whereas the velocity head is no more than 5 mm in chemical or gas treating columns. It is true, however, that when the trays of a 50 tray tower are replaced by packing, the pressure drop through the column is typically reduced by a full order of magnitude, to wit: on the order of 30 mmHg. This is especially true of structured packing such as that set forth and described in U.S. Pat. No. 4,604,247 assigned to the assignee of the present invention. If the kinetic energy of the feed vapor is kept at 10 mm or more, severe mal-distribution will occur. Since it is not usually economical to oversize the vapor nozzle or to enlarge the existing one, a well designed vapor distribution system is absolutely essential for smooth start up and operation.
There are two conventional vapor nozzle designs for crude oil atmospheric or vacuum process towers, to wit: tangential and straight run. When atmospheric and vacuum type towers are addressed, special care is taken to handle the large volume of vapor. Tangential baffles or vapor horns have been incorporated in the prior art. In these configurations, the vapor flow is directed through a housing or horn, and deflected by the baffles prior to ascension through the column. In a straight run configuration, gravity is used to effect the separation of vapor from the solids and liquids that are discharged beneath the vapor trajectory. Upward vapor distribution may also be improved across the column section by utilizing a tapered channel with preset diffuser vanes. With either configuration an additional vapor distribution tray above the feed nozzle may be necessary to insure uniform distribution of the ascending vapor.
Both single and multiple vapor nozzles may be utilized for process columns. Vapor feed nozzles have, for example, been placed 180 degrees apart as well as 90 degrees apart around the circumference of a column. For either configuration, a properly designed vapor distributor is a requirement and select patterns of diffuser vanes may be utilized. With such a configuration it may be necessary to again place an additional vapor distribution tray above the entrance nozzle to insure uniform distribution. This is particularly true of configurations adapted for handling two phase flow. In heavy oil refining, contamination of the vapor from the bottom black oil must be minimized.
It is also conventional in the prior art to use a cylinder having a diameter less than the diameter of the tower for securing a structured packing in the lower region of the tower in which a packed wash oil system can be installed. In many instances tower space outwardly of the packed wash oil system is wasted due to the tower design and the use of conventional vapor diffusers disposed therebeneath. It is critical in any tower designed for maximum efficiency to effectively utilize all tower height for mass and heat transfer. For this reason any tower design that increases the available packing height will generally improve tower performance. It would be an advantage therefore to provide a system which maximizes the available height within the tower for packing to improve performance and to gain additional vessel space for additional packing or for an additional side cut. The present invention provides such an advance over the prior art by utilizing an annular vapor horn housing disposed within the tower and inwardly of which is secured a packing bed to thereby gain usable vessel height for mass transfer therein, as well as improving the initial vapor distribution upwardly therethrough.